1. Field of the Invention
The present invention relates generally to position determination systems and, in particular, to providing timing information to a wireless device in a position determination system.
2. Description of the Related Art
Systems for determining the geographic position of a wireless device are well known in the art. A commonly used position determination system is the Global Positioning System (GPS) operated by the United States Department of Defense. The GPS includes a network of 24 satellites that orbit the earth in six circular planes. The GPS satellites are spaced so that, at any given time and from any geographic position, at least five GPS satellites will be above the horizon. Each GPS satellite continually transmits its current time and current position for reception by GPS receivers on earth. A GPS receiver may determine its geographic position by locating the signals transmitted from four of the GPS satellites that are in view. The difference between each signal's transmission time and reception time is used to calculate the distances (i.e., pseudoranges) between the GPS receiver and each of the four GPS satellites. These distance measurements, along with the satellite position and time information received through the GPS signals, are used by the GPS receiver to synchronize its internal clock with the GPS clock and calculate its longitude, latitude and altitude with an accuracy of less than 100 meters.
The GPS satellites transmit the GPS signals on the 1575.42 MHz carrier frequency using direct spread spectrum modulation. Each GPS signal is modulated by a pseudorandom noise (PN) code that uniquely identifies the transmitting GPS satellite. Each PN code is a repeating sequence of 1023 chips that is transmitted at a rate of 1023 chips per millisecond. To locate a particular GPS signal, the GPS receiver generates a replica of the associated 1023 chip PN code sequence and searches for a matching 1023 chip sequence in a received signal. The GPS signal will be found if the GPS receiver is able to align the replica, chip-to-chip, with a 1023 chip sequence in the received signal. Many GPS receivers include correlator hardware that allows a range of contiguous chip positions (e.g., 32 chips) to be searched in parallel, thereby reducing the time needed to search across each of the 1023 chip positions. In addition, due to Doppler shift, the perceived carrier frequency of each received GPS signal will vary depending on the relative positions of the GPS receiver and the transmitting GPS satellite. Because the relative positions of the GPS satellites and GPS receiver are typically unknown, the search for a GPS signal may further require searching across a range of potential carrier frequencies.
An exhaustive search across the entire range of potential carrier frequencies for each of the 24 PN codes can be undesirably time consuming—in many cases taking minutes. To reduce the search time, GPS receivers are often provided with aiding information that may be used to help identify the GPS satellites that are in view and the associated Doppler frequencies of each associated GPS signal before commencing the search. In one approach, the GPS receiver stores almanac data describing the approximate orbits of the GPS satellites. Using the almanac data, its internal clock and an estimate of its current position, the GPS receiver calculates the approximate positions of the GPS satellites and identifies which of the 24 GPS satellites are likely to be in view. Next, the GPS receiver estimates the Doppler shift of each GPS signal transmitted from the identified GPS satellites. A search for a GPS signal may then be conducted, focusing on the PN codes of the identified GPS satellites and centered about the expected Doppler frequencies. Although the use of almanac data can reduce the search time, the use of almanac data lacks the precision and efficiency of other known approaches.
In one known approach, the GPS signal acquisition time is reduced through the use of a position determination entity (PDE). A PDE is typically part of a wireless communications network that includes a plurality of base stations and at least one mobile device. The PDE continually tracks the positions of the GPS satellites through a network of stationary GPS receivers distributed across the coverage area of the wireless communications network. Before searching for the GPS signals, the mobile device transmits a request for GPS aiding information to the PDE through a local base station. Using the identity of the local base station, the PDE determines the approximate location of the mobile device and provides the mobile device with the identities and positions of the GPS satellites that are likely to be in view, and the expected Doppler shift of each identified GPS signal. The real-time information compiled by the PDE is typically more precise than standard almanac data and often results in a shorter GPS signal acquisition time.
The GPS signal acquisition time has been reduced even further in code division multiple access (CDMA) networks. In a CDMA network, each base station maintains a clock that is synchronized with the GPS time and transmits a timing beacon to mobile devices in its coverage area. The mobile devices use the timing beacons to synchronize their internal clocks with the base station's clock, often with an accuracy of less than 4 microseconds. The PDE and stationary GPS receivers also maintain clocks that are synchronized with the GPS time. In operation, the stationary GPS receivers track the times at which the beginning of each PN frame (i.e., 1023 chip PN code sequence) is received by the stationary GPS receiver. The PDE transmits the identities of the GPS satellites in view to the mobile device, along with the associated Doppler shift and associated PN frame reception time of each GPS signal. The mobile device may use this information to identify the GPS satellites in view, the expected reception frequencies of the associated GPS signals, and the times at which the associated PN frames are expected to be received at the stationary GPS receiver. Using the received GPS timing information, the mobile device is able to quickly align the generated 1023 chip PN code sequence with a matching 1023 chip sequence from the received GPS signal.
In many wireless communications networks, however, such as asynchronous wideband code division multiple access (WCDMA) networks and Global System for Mobile Communications (GSM) networks, the internal clocks of the mobile devices are not necessarily synchronized with the base stations, PDEs and stationary GPS receivers. Consequently, a mobile device cannot rely on information that describes the time at which a stationary GPS receiver expects to receive the beginning of each PN frame. Transmitting such GPS timing information to the mobile device would provide little benefit in terms of reducing the GPS signal acquisition time.
In view of the above, there is a need for an improved system and method for providing timing information to a wireless device in a position determination system to assist the wireless device in acquiring GPS signals.